The top U.S. military commander has briefed Congress on the possibilities and costs of military intervention in Syria.

In a letter released on July 22, the Joint Chiefs of Staff chairman, General Martin Dempsey, outlined five options.

They include providing nonlethal intelligence and weapons training to the opposition, launching limited strikes from outside the country, enforcing no-fly zones and buffer zones, and launching an operation to "assault and secure" the Syrian regime's chemical weapons.

The letter, addressed to Senate Armed Services Committee chairman Carl Levin, also warned of risks such as retaliation by the regime of Bashar al-Assad and the empowerment of radical Islamists.

Dempsey said a decision to intervene militarily in Syria is "a political one."

U.S. officials announced last month that they would send military aid to Syrian rebels.